1. Field of the Invention
The present invention relates to an improved process for fabricating metal oxide materials and devices made therefrom via the metallo-organic decomposition method.
2. Description of Related Art
A new class of layered ferroelectric materials having a layered perovskite structure have begun finding use as thin ferroelectric films in radiation-hard, non-volatile microelectronic memories, high dielectric constant capacitors, energy storage devices, and the like. These layered materials comprise complex oxides of metals such as strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, haffium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium, and thallium that spontaneously form layered crystalline lattices that include alternating layers of distinctly different sublattices, such as ferroelectric and non-ferroelectric sublattices. Generally, each layered material will include two or more of the above metals. For example, strontium, bismuth, and tantalum form the layered material strontium bismuth tantalate, SrBi.sub.2 Ta.sub.2 O.sub.9 (SBTO).
A variety of techniques has been disclosed for fabricating such layered ferroelectric materials; see, e.g., U.S. Pat. Nos. 5,434,102 and 5,439,845, both issued to H. Watanabe et al, which are directed to metallo-organic decomposition (MOD) to form the layered ferroelectric materials. The coated wafer is baked to dry and decompose the precursor and annealed to form a layered material on the wafer. Each metal in the ferroelectric material is formed separately as a carboxylate and the carboxylates are combined prior to application to the wafer. The carboxylates may be formed (in the case of strontium) by reacting the metal with a carboxylic acid. For the case of the other metals, these may be formed by reaction of a carboxylic acid with a metal alkyl or aryl, or with a metal alkoxide, or more preferably, with the acid anhydride and metal alkoxide; see, application Ser. No. 08/669,122, filed Jun. 24, 1996, now U.S. Pat. No. 5,721,009
An important application of the layered ferroelectric materials includes ferroelectric capacitors (see, e.g., C. A. Paz de Arujo et al, "Fatigue-free ferroelectric capacitors with platinum electrodes", Nature, Vol. 374, pp. 627-629 Apr. 13, 1995!). A problem associated with these capacitors, and with thin film metal oxide dielectrics in general, is an unacceptably high leakage current.
Previous work in the art of ferroelectric materials has concentrated on the bismuth layered structure SrBi.sub.2 Ta.sub.2 O.sub.9 (SBTO). It has been shown that this material can produce devices with much improved retention of polarization and fatigue (useful number of cycles of polarization) as compared to other ferroelectric metal oxides (for example, lead zirconium titanate PZT!). Symetrix Corporation and its licensee, Ku-jundo Chemical of Japan, offer for sale a liquid source and process to construct SBTO material and devices. However, the inventors of the present invention have found that the supplied chemicals and process do not give devices with consistent electrical properties nor devices with sufficient properties at elevated temperatures.
In the patents and publications listed above, Symetrix Corporation describes liquid source materials and a process to produce bismuth-layered ceramic thin film ferroelectric devices. Both Symetrix, and their licensee (Kojundo Chemical) offer for sale the liquid source to the material, named "Y1", which is disclosed as the bismuth-layered structure SrBi.sub.2 Ta.sub.2 O.sub.9. According to the Symetrix disclosures, a liquid source containing a metal salt--organic acid mixture having the stoichiometry of Bi.sub.2.18 Sr.sub.0.99 -Ta.sub.2.00 gives on processing the desired Y1 material. However, in the experience of the present inventors, the liquid source of Y1 supplied by either Symetrix or Kojundo does not on processing always give Y1 ferroelectric films of consistent electrical quality. Also, the Y1 thus formed, even in its best examples, does not have sufficient high temperature performance to meet the critical needs of the military and space systems hardware electronics applications.
In light of the foregoing, the present inventors disclosed in application Ser. No. 08/635,313, filed Apr. 19, 1996, now abandoned a process for providing oxide materials having a composition described by the formula SrBi.sub.2.00 Ta.sub.2.00-x,Nb.sub.x O.sub.9 (SBTNO) where x ranges from 0.00 to 2.00 and is controlled to within about 1%, having improved high temperature performance. A batch of the desired composition is initially prepared by conventional processes to provide a nominal composition that may be close to the desired composition, but not the exact desired composition. A portion of the batch is processed to form a thin film, the stoichiometry of which is then analyzed. The batch composition is then modified by adding thereto a quantity of one or more of the constituents to bring the batch composition to the desired stoichiometry. Further improved high temperature performance is achieved with values of x within the range of about 0.10 to 1.9, and more preferably, about 0.4 to 0.7, and most preferably about 0.56. The thus-modified metal organic acid salt solutions provide ceramic thin films having improved electrical properties (particularly consistent properties from batchto-batch, improved ferroelectric remnant polarization, and much improved high temperature performance) as compared to the thin films prepared from unmodified solutions.
Although the process provides improved electrical properties, it has been subsequently discovered that ferroelectric thin film capacitors made by the process evidence leakage currents that are capable of being improved by the process of the present invention.